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Biol 121 225 Freeman 243-55 (Ch. 12)

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BIOL 121
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Biol 121 225 Freeman 243-55 (Ch. 12) Jan. 29, 10 Define... fertilization. -process of uniting sperm and egg (in sexual reproduction [SR]) Embryo -cells in a newly growing offspring -all the cells are products of mitotic divisions from the first nucleus that formed at fertilization Gametes -reproductive cells: sperm or egg -haploid (1n) – 1 set of chromosomes Define meiosis -nuclear division that leads to a halving of chromosome number -precedes formation of eggs and sperm in animals Sex chromosome, autosome -sex chromosome is the chromosome associated w/ sex of the individual -autosomes are non-sex chromosomes Homologous chromosomes -“same proportion” (same size + shape) -are pairs of sex chromosomes or autosomes -also known as homologs -homologous chromosomes carry the same genes (but may have diff. alleles) Define gene -section of DNA that influences one or more hereditary traits in an individual Allele -different versions of the same gene Karyotype -number and types of chromosomes present Diploid -“double-form” -have two versions of each type of chromosome -diploid organisms have two alleles of each gene – one on each of the homologous pairs of chromosomes Chromosome number in -chromosome number in each cell is reduced during meiosis meiosis -results in haploid cells Brief summary of meiosis I -the homologs in each chromosome pair separate from each other -one homolog goes to one daughter cell; the other homolog goes to the other daughter cell -therefore, diploid (2n) becomes (n) in daughter cells of M1 Brief summary of M II. -sister chromatids from each chromosome separate, each going to one daughter cell -cells produced by M II also have one of each type of chromosome, but now the chromosomes are unreplicated Zygote -when two gametes fuse during fertilization, a full complement of chromosomes is restored -the cell that results from fertilization is diploid and is called a zygote -homologous chromosomes are referred to as either maternal or paternal in origin Synapsis and tetrads -during early prophase I, homologous chromosome pairs come together -this pairing process is called synapsis -happens because regions of homologous chromosomes that are similar at the molecular level attract one another -structure that results from synapsis is called a tetrad -a tetrad consists of 2 homologous chromosomes, with each homolog consisting of two sister chromatids -the chromatids from the homologs are non-sister chromatids Chiasmata -plural of chiasma -during late prophase I, non-sister chromatids begin to separate at many points along their length but stay joined in certain locations and look like they cross over one another,
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